McCartney grew up as a supporter of Heart of Midlothian. Having played international football for the Scottish Schoolboys he turned down the opportunity to join Hearts as a trainee and instead joining West Bromwich Albion, for whom he signed on as an apprentice in July 1970. Although he signed a professional contract in December 1971, he failed to break into the first team.

In May 1973, he joined his former manager at Albion, Alan Ashman, signing for Second Division Carlisle United. At the end of McCartney's first season Carlisle finished in third place in the Second Division and were promoted to the First Division for the first time in the club's history. The Cumbrian club only managed one season in the top division, in which they finished in last place following which Carlisle fell down through the divisions and by 1977 they were in the Third Division.

In July 1980, McCartney returned to the First Division when Southampton's manager Lawrie McMenemy signed him for £50,000. He was one of two players who made his debut for the Saints in a 2–0 victory against Manchester City on 16th August 1980, the other being England international Kevin Keegan, whose arrival overshadowed that of McCartney.

McCartney failed to settle back in the First Division and, after a run of 15 consecutive matches he lost his place to Nick Holmes.

McCartney returned to the Third Division with Plymouth Argyle in August 1981, for the same fee that Southampton had paid to Carlisle a year earlier.

McCartney made 62 appearances for Argyle before returning to Carlisle just before the end of the 1982–83 season in an exchange deal for Gordon Staniforth.

He remained at Carlisle for five years during which time they were relegated twice and in 1987–88 they finished second from bottom of the Fourth Division.

In February 1986, McCartney suffered an ankle injury in a match at Sunderland which brought his Football League career to an end, aged 33.

In the summer of 1988, McCartney returned to Scotland joining Gretna then of the Northern League Division One initially as a player-manager, becoming full time manager the following year.

In his first season as manager, Gretna finished as runners-up in the Northern League, followed by the championship in the next two years. In 1991–92, Gretna became the first Scottish club to reach the First Round proper of the F.A. Cup for 105 years, where they were eliminated by Rochdale after a replay. They reached the first round again in 1993–94, going out 2–3 to Bolton Wanderers of the First Division.

Following their success in the Northern League, Gretna applied to join the Scottish League and McCartney himself worked on the required ground improvements. Following the failure of the application, and against McCartney's wishes, Gretna joined the Northern Premier League for the 1992–93 season, where they finished in sixth place.

McCartney was concerned that the additional costs of travelling to clubs across northern England and North Wales would cause the club financial difficulties.

Gradually the club's performances deteriorated with finishes in the lower half of the league and the club were forced to the edge of insolvency, leading to McCartney being dismissed in January 2000.

Gretna was later acquired by Brooks Mileson who invested heavily in the club. In 2006, they reached the Scottish Cup Final, where they were defeated on penalties by McCartney's boyhood heroes, Hearts.

Although Gretna defender Derek Townsley supplied him with a complimentary ticket to the Cup Final, McCartney supported Gretna's opponents.

McCartney continued to watch Hearts regularly, travelling from his home in Carlisle, where he worked in the building trade and as a van driver.

I had the pleasure of meeting Mike McCartney once in the bar of the old South Stand at Eastville Stadium [Bristol Rovers] after a match against Carlisle United. I was sat alone at a table, when he came and sat opposite me and engaged me in conversation. Forty years later I can still recall that chat and he left a good impression of being a really nice chap. R I P